The objective is to define the mechanisms of toxic action and selectivity for nicotinoid insecticides and thereby insure their effective use with minimal risk to humans and the environment. Synthetic nicotinoids are the only major new class of insecticides of the past 4 decades. Recently imidacloprid (IMI) has become the main pest control agent for many crops and the associated residues are now a dietary contaminant for humans. The nicotinoid insecticides are related to nicotine in their structure and action at the nicotinic acetylcholine receptor (nAChR) and at least one of the IMI metabolites and several of the candidate insecticides are similar to nicotine in toxicity (LD50 in mice). The first hypothesis is that the nAChR binding-site specificity for nicotinoid insecticides is related to receptor subtype, function, neuronal region, and developmental stage. The major alpha4beta2 subtype and the alpha7-containing receptors are emphasized, using [H3]nicotinoid insecticide radioligands versus [H3]nicotine and [125I]alpha-bungarotoxin in studies of binding site pharmacological profiles, antibodies to isolate subunits from brain and cultured cells, functional assays monitoring 86Rb+ efflux, binding site distribution in brain by autoradiography and changes during development utilizing cultured hippocampal neurons. The second hypothesis is that the metabolic lability of the nicotinoid insecticides leads to bioactivated toxicants such as desnitro-IMI in mammals. The goal is to determine the extent to which metabolic activation and detoxification by P450s and flavin-containing monooxygenases confer selective toxicity among the nicotinoids with emphasis on IMI. The third hypothesis is that nicotinoid insecticide selectivity depends in large part on major structural differences in the neuronal nAChR binding sites of mammals and insects. Optimized nicotinoid insecticide probes will allow isolation (affinity chromatography) and photoaffinity radiolabeling of the toxicologically relevant nAChRs (mammal and insect). The goal will be to define the binding site as to subtype, subunit, peptide sequence and ultimately amino acid derivatized. These multiple approaches will establish mechanisms for nicotinoid selective toxicity in mammals and insects.